This invention relates to methods and apparatus for identifying faulted phases on an electric power transmission line.
According to statistics, most faults which occur on a high voltage electric power transmission line last for a fraction of a second only, and 93% of faults which occur are single phase to ground faults. A common practice on major high voltage transmission lines is therefore to use single pole reclosing, that is, when a fault exists on a single phase only which can be identified, that single phase is disconnected and reconnection attempted after a short delay by which time the fault has often cleared. If reconnection is unsuccessful, i.e. the fault is still present when reconnection is attempted, the reclosure sequence may be repeated, e.g. two or three times. By using this practice disturbance to an electric power supply system of which the transmission line forms part is reduced and hence the reliability of the system is improved.
It is at present possible to identify with reasonable certainty the faulted phase when single phase fault occurs on a single circuit transmission line. It is also possible to identify the faulted phase when a single phase fault occurs on one of the two circuits of a double circuit transmission line. A scheme is described in EP-A-0325786 which is intended to identify the faulted phases when single phase faults occur simultaneously on both circuits of a double circuit transmission line, but this scheme requires measurement of quantities at both ends of the transmission line, and hence requires communication between the fault identifying apparatus and both ends of the line.